CARAJÀS, BRAZIL—The remote caves of Carajàs contain some of the earliest evidence of human settlement in the Amazon, but they are threatened by plans to expand a mining operation that extracts iron ore for export. Archaeologists and speleologists are surveying the caves, since at least 24 of them will probably be destroyed. “This is a crucial moment to learn about the human history of the Amazon, and by extension the peopling of the Americas,” said Brazilian caver and historian Genival Crescêncio.